Black mystery stains

[captjohn]

I have 2 large and several small ones. The large ones are quarter sized. I must have some small chips in the gelcoat where it starts because it's always the same places. They flare out with water flow. I can spot treat with citric acid and they fade or go away but come back in a month.
Water is crystal clear. Any ideas?
fc-6
cc-0
ph-7.4
ta-150
ch-300 (was at 150)
cya 40
borates-30

 

[Bama Rambler]

Those look like stains from exposed metal. Especially given the fact that they always come back in the same places.

 

[captjohn]

Those look like stains from exposed metal. Especially given the fact that they always come back in the same places.

Don't know what metal would be in my fiberglass. I have the 2 larger areas and 10-15 very small ones. I figured it was some type of reaction with chems and pin holes through the gel coat. Just not sure what. At least I can get rid of them. I'm going to try a plastic bag on a pole with a very small hole filled with citric acid and hold the hole over the areas. Should work and use much less acid to rid the stain.
I'm still listening.
Thanks

 

[Bama Rambler]

Something could have gotten under the gelcoat during mfg but that doesn't seem likely.

 

[eripaulie]

I have the same black dots with the flare out "tails" that you do. I have several but they are only as big as the smaller one in your bottom picture. Have never figured out what they are. I have tried to find pictures of what cobalt staining looks like but haven't been successful. They are on the bottom of the pool and the sides. I did try a vitamin c tablet on some on the ones on the sides, they fade for only a moment then reappear. I will be following this thread to see if you come up with anything.

 

[JamesW]

Most likely to be cobalt staining. Cobalt is used as one of the chemicals in some fiberglass resin. A reducing agent, such as ascorbic acid or oxalic acid might help temporarily, but the stains are likely to reappear. Using a higher calcium level and a sequestrant might help reduce reoccurrences. However, I think that the pool would need to be refinished with a high quality gelcoat to prevent future reoccurrences.

Of all the raw materials used to manufacture fiberglass pools, gel coat is by far the most expensive per pound. If a manufacturer wants to cut costs, this is a good place to start, because on a brand new pool, ordinary gel coat looks just exactly like the best gel coat that money can buy. But just wait 24 or 36 months and you will see a big difference. Premium gel coat won’t produce water blisters and black cobalt stains, and ordinary gel coat will.

http://www.sanjuanpoolsok.com/manufacturing-process/